1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a digital multi-functional system including a combination thereof, and more particularly, to a transfer device that contacts an image bearing member and an image forming apparatus including the transfer device.
2. Description of the Background Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member; an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
There are two types of known color-electrophotography image forming apparatuses. One is a single-drum color image forming apparatus that includes one photoconductive drum around which a plurality of developing devices designated for different colors of toner is disposed. A composite toner image is formed on the photoconductive drum with toner by the developing devices.
The other type of known color-electrophotographic image forming apparatus is a so-called tandem-type image forming apparatus, in which a plurality of photoconductive drums, one for each of the colors black, cyan, magenta, and yellow, is arranged in tandem and each provided with a developing device. Multiple toner images of a respective single color are formed on the photoconductive drums. Then, the toner images are transferred onto an intermediate transfer member or a recording medium so that they are superimposed one atop the other, thereby forming a composite color toner image.
Alternatively, in the tandem-type image forming apparatus, the toner images formed on the plurality of the photoconductive drums may be temporarily transferred onto the intermediate transfer belt by a plurality of primary transfer rollers, and then ultimately onto the recording medium by a secondary transfer roller.
In such a tandem-type color-electrophotography image forming apparatus, an intermediate transfer device including the primary transfer rollers usually has a long horizontal axis because the plurality of the photoconductive drums are arranged linearly. Moreover, to enhance speed and durability of the image forming apparatus, a diameter of the photoconductive drum is increased. Consequently, the total length of the intermediate transfer device increases.
In view of the above, in order to maintain strength of the intermediate transfer device, a housing and parts that support each device in the intermediate transfer device are made of steel plates.
In one example of such an intermediate transfer device, the plurality of primary transfer rollers that contacts the respective photoconductive drums is supported by a single swingable arm serving as a roller support member and pressed against the photoconductive drums by compression springs. Rotation of the swingable arm about a shaft enables the primary transfer rollers to contact or separate from the photoconductive drums.
Although advantageous and generally effective for its intended purpose, there is a drawback to this configuration in that the distance from a point of support of the swingable arm to a point of load tends to be long, thereby degrading the strength of the swingable arm.
In this configuration, rotation of the roller support member enables the primary transfer roller to contact the photoconductive drum. In order to achieve stable contact between the primary transfer roller and the photoconductive drum, it is important to prevent the electric connection from the power source to the primary transfer roller from interfering with rotation of the roller support member.
Furthermore, since a high voltage in a range of 1 [kV] to 10 [kV] is generally applied to the primary transfer roller to transfer the toner images on the photoconductive drum, shielding ability is also desired to prevent leak current.
At the same time, however, since the primary transfer roller rotates together with the swingable arm, that is, the roller support member, reliable electric connection needs to be maintained even when the position of the electrical contact at the primary transfer roller side moving together with the roller support member changes relative to the electrical contact at the power source side fixed to the main body.
In view of the above, a wire harness is used to connect loosely the contact subjected to move and the contact fixed to the main body, in which the wire harness is not stretched but retains slack. Alternatively, a coil spring is used to connect the contact subjected to move and the contact fixed to the main body.
In a related-art image forming apparatus, the coil spring is used to electrically connect a roller support shaft that rotates together with a rotary member and a voltage application device fixed to the main body. In this configuration, the coil spring serves as the voltage application path between the roller support shaft and the voltage application device. According to this configuration, even when the rotary member rotates, moving the contact at the roller support shaft side, elasticity of the coil spring allows the coil spring to deform in accordance with the displacement of the contact at the roller support shaft side relative to the contact at the voltage application device side, thereby maintaining electric connection between the roller support shaft and the voltage application device.
Although advantageous, when using connecting parts such as the wire harness and the coil spring to connect the contact subjected to move and the contact fixed to the main body, the weight of connecting parts and resilience act in the direction causing the roller support member to rotate, hence changing the contact pressure of the primary transfer roller. For example, in the case of the wire harness, the weight of the slack wire harness acts on the contact of the roller support shaft, moving the contact down and hence changing the contact pressure. In the case of the coil spring, the coil spring twists undesirably, moving the contact of the roller support shaft as well.
Furthermore, since the majority of parts constituting the intermediate transfer device are made of metal plates, the size and the number of parts needed to prevent leak current at the point of support of the spring increase, thereby complicating efforts to reduce cost and facilitate assembly.